WASHINGTON — The Bush administration on Friday softened its hardline stance on how to thwart Iran’s suspected nuclear arms program, agreeing to support a European plan that offers economic incentives for the Tehran government to give up any weapons ambitions.

Secretary of State Condoleezza Rice signaled, however, that Iran should move quickly or face the threat of harsh United Nations Security Council sanctions. The administration also privately expressed skepticism that Iran would live up to the bargain.

Until now, the administration has insisted that Iran deserves no reward for simply abiding by an international arms compact that forbids nuclear weapons development. The United States suspects Iran is using a legitimate program to develop nuclear power plants as cover for illegal weapons development.

“I’m pleased that we are speaking with one voice with our European friends,” President Bush said during a trip to Shreveport, La. “I look forward to working with our European friends to make it abundantly clear to the Iranian regime that the free world will not tolerate them having a nuclear weapon.”

The United States agreed to drop opposition to Iranian membership in the World Trade Organization and to allow some sales of spare parts for civilian aircraft.

If that carrot does not work, the Europeans agreed to support use of the stick the United States has unsuccessfully sought before: U.N. sanctions.

Rice said there is no timetable for negotiations but added, “This has been going on for some time.”

“I would think that if the Iranians are going to demonstrate that they are prepared to live up to their obligations, that they would want to do that sooner rather than later,” she said after meeting at the State Department with Ukrainian Foreign Minister Borys Tarasyuk.

“The Iranians need to take the opportunity that the Europeans are presenting them,” Rice said.

“We shall have no choice but to support referring Iran’s nuclear program to the U.N. Security Council,” a confidential EU document obtained by The Associated Press said.

There was no immediate response from Tehran.

It remains an open question whether Iran will surrender its right to both enrich uranium and reprocess it, said a senior administration official who briefed reporters on the condition of anonymity.

Still, the official said, the U.S. has made a move that could help the Europeans in their negotiations with the Iranians.

The shift puts the United States side by side with British, French and German diplomats. Just weeks ago the Bush administration seemed to write off their talks with Tehran as fruitless.

The change came about as Bush and Rice received personal assurances that the European countries negotiating with Tehran over its nuclear program are firmly committed to stopping any weapons program there, administration officials said Friday.

"There is a general recognition that we don't need these military-style weapons in New Zealand, so it's very easy to win cross-party support for this," said Mark Mitchell, who was defense minister in the previous, center-right government and who supports the ban initiated by the center-left-led Labour Party.